a. Field of the Invention
This invention relates to the production of flaked metal powders, especially those having a narrow particle size distribution, whiter color, and a very high sparkle effect. More particularly, the invention relates to aluminum, nickel, stainless steel, brass, cupro nickel, and bronze powders having the above characteristics.
B. Description of Prior Art
In my U.S. Pat. No. 3,995,815, entitled "PRODUCTION OF FLAKED METALLIC POWDERS" there is described a method of making these powders in which the ratio of attritive elements to finely divided metal is between 37:1 and 10:1 by weight. As a preferred condition, the ratio of inert liquid to finely divided metal is between 0.5:1 and 1:4 by weight and the ratio of finely divided metal to lubricant is between 30:1 and 1:1 by weight. Although this process has been found to be quite efficient, it is not possible to produce "flaked metal powders" with a narrow particle size distribution, an improved whiteness, and a very high sparkle effect as required in today's applications, such as in decorative finishes, automotive and appliance applications, paints, inks, plastics, and the like. Recently, there has been disclosed in U.S. Pat. No. 3,776,473 and its division U.S. Pat. No. 3,901,688, that it is possible to produce aluminum flaked powders with high specular reflectivity by the wet ball milling process. The process is carried out using grinding balls to powder a volume ratio which varies between about 15:1 and 75:1 and grinding balls to milling liquid volume ratio in the range of about 2:1 to about 1:1.25. This process is very uneconomical, time consuming and, although it produces powders of high sparkle, it has been found that its brightness is not sufficient in that when the pigment is treated, the powder is not sufficiently white. Furthermore, the size distribution is not narrow enough to fully satisfy modern requirements, such as in the automotive paint industry.
It has also been found that while the ratios of ingredients mentioned in U.S. Pat. No. 3,776,473 may be useful for tube mills, the products obtained with the equipment described in my U.S. Pat. No. 3,995,815 using the ratios defined in U.S. Pat. No. 3,776,473 are of very limited value because the fineness range makes them unacceptable.